Method of making polymer compositions containing thermoplastic starch

ABSTRACT

There is provided a method of preparing a thermoplastic starch and synthetic polymer blend, said method comprising the steps of: (a) providing a starch suspension comprising starch, water and a plasticizer, preferably glycerol; (b) obtaining a thermoplastic starch from the starch suspension by causing gelatinization and plasticization of said starch suspension by exerting heat and pressure on said starch suspension in a first extrusion unit; (c) evaporating and venting off residual water from said thermoplastic starch to obtain a substantially moisture-free thermoplastic starch; (d) obtaining a melt of a synthetic polymer or polymer blend in a second extrusion unit; and (e) combining the melt obtained from step (d) with the substantially moisture-free thermoplastic starch. Also provided are compositions of matter comprising immiscible blends of thermoplastic starches, polymers, and compatibilizers.

This is a Divisional Application of U.S. Ser. No. 09/472,242, filed Dec.27, 1999, now U.S. Pat. No. 6,605,657, the complete disclosure of whichis incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to polymer compositions containing thermoplasticstarch and to a method of making these compositions.

2. The Prior Art

The blending of starch with synthetic polymers such as polyethylene andpolypropylene has been the subject of increasing interest over recentyears. The motivation is keen since starch is an abundant andinexpensive filler material. Moreover, starch may also impart partialbiodegradability to the resulting blend.

Natural starch found in plant products can be isolated as a granularpowder. It is known that natural starch can be treated at elevatedtemperature and pressure with addition of defined amounts of water toform a melt. Such a melt is referred to as gelatinized or destructurizedstarch. It is also known to mix destructurized starch with additivessuch as plasticizers to obtain a thermoplastic starch or TPS. It isknown to mix these forms of starch with synthetic polymers andco-polymers. For example, U.S. Pat. No. 5,095,054, and Ind. Eng. Chem.Prod. Res. Dev. vol 19, p. 592 (1980) describe such a process.

Difficulties have arisen in that the presence of starch has had anegative impact on the physical properties of the resulting mixture whencompared to the pure synthetic polymers. Furthermore, when starch ismixed with synthetic polymers or co-polymers, the starch domains areenveloped by the non-biodegradable synthetic polymers and consequentlytheir biodegradability is significantly reduced.

A biodegradable material can be defined as one that is able to beconverted to CO₂ and H₂O by certain common microorganisms. It is furtherunknown in the art to achieve mixtures of starch with non-biodegradablepolymers where the starch domains are readily accessible forenvironmental degradation while still maintaining good mechanicalproperties.

With respect to the method of preparing polymer and TPS blends, someblending studies have been reported using internal mixers. Examples ofsuch studies are found in international application WO 90/14388,European Patents 0 554 939 and 0 327 505.

It is also known from the article entitled “Processing andcharacterization of thermoplastic starch/polyethylene blends”, publishedin Polymer, 38 (3), 647 (1997), to blend TPS and low densitypolyethylene (LDPE) in a continuous process using a co-rotatingarrangement of a twin-screw extruder fed on one side by a single-screwextruder. The side extruder is used to prepare the TPS. The mainextruder is used to prepare the LDPE melt which is combined with the TPSmelt. However, such process results in TPS/LDPE blends having poorphysical properties including the presence of water and of bubbles.Moreover, tensile properties of the extrudate dropped off dramaticallyat about 10% or more of TPS content. Tests revealed that the TPS,present as a dispersed phase in the extrudate, exhibited spherical orellipsoidal shapes. Consequently, the extrudate was not easilybiodegradable since the great majority of spherical or ellipsoidalshapes were enveloped in polyethylene which is not biodegradable. Inother words, the dispersed TPS phase was not continuous.

The prior art is also silent on controlling process parameters toachieve controlled morphologies of the resulting blend.

Thus, it is an object of the present invention to provide a novel methodfor obtaining TPS/polymer blends having controllable and improvedphysical properties over the prior art blends.

It is a further object of the present invention to provide an improvedproduct comprising a blend of TPS and polymer(s) having improvedphysical properties over prior art blends.

It is a related object of the invention to provide an improved productwherein the TPS phase is continuous so as to allow biodegradationprocesses to take place within the product.

In preferred embodiments, the product is extruded sheet or blown film.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that this detaileddescription, while indicating preferred embodiments of the invention, isgiven byway of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art.

SUMMARY OF THE INVENTION

There is provided a method of preparing a thermoplastic starch andsynthetic polymer blend, said method comprising the steps of: (a)providing a starch suspension comprising starch, water and aplasticizer, preferably glycerol; (b) obtaining a thermoplastic starchfrom the starch suspension by causing gelatinization and plasticizationof said starch suspension by exerting heat and pressure on said starchsuspension in a first extrusion unit; (c) evaporating and venting offresidual water from said thermoplastic starch to obtain a substantiallymoisture-free thermoplastic starch; (d) obtaining a melt of a syntheticpolymer or a polymer blend in a second extrusion unit; and (e) combiningthe melt obtained from step (d) with the substantially moisture-freethermoplastic starch.

Also provided are compositions of matter comprising immiscible blends ofthermoplastic starches and polymers. The compositions of matter exhibitfavorable mechanical properties and provide a continuous or highlycontinuous thermoplastic phase so as to enhance the biodegradability ofthe composition of matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration, top view, of the apparatus used ina preferred embodiment of the method of the present invention;

FIG. 1B is a side view;

FIGS. 2 a)-2 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE/TPS blend (ca. 30% TPS): (a) LDPE2040/TPS (20% glycerol)500X; (b) LDPE2049/TPS (20% glycerol) 500X; (c) LDPE2040/TPS 27.5%glycerol) 1000X; (d) LDPE2049/TPS (27.5% glycerol) 500X;

FIGS. 3 a)-3 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE2040/TPS (27.5% glycerol) blends cryogenically fractured inthe axial direction: (a) 29% 1000X; (b) 35.5% 1000X; (c) 44.7% 1000X;and (d) 53.3% 500X;

FIGS. 4 a)-4 d) are scanning electron micrographs (SEM) of variousembodiments of the product of the present invention wherein the productis an LDPE2040/TPS (27.5% glycerol) blends cryogenically fractured inthe transversal direction: (a) 29% 1000X; (b) 35.5% 1000X; (c) 44.7%1000X; and (d) 53.3% 500X;

FIG. 5 is the accessibility of starch domains in LDPE/TPS blends after96 hours of extraction;

FIGS. 6 a)-6 b) is the relative elongation at break (ε/ε₀) of LDPE/TPSblends: (a) LDPE2040; (b) LDPE2049;

FIGS. 7 a)-7 b) is the relative Young's Modulus (Ε/Ε0) of LDPE/TPSblends: a) LDPE2040; b) LDPE2049;

FIG. 8 is the relative Young modulus (Ε/Ε₀) of LDPE2040/TPS blends(27.5% glycerol in slurry);

FIG. 9 is the relative elongation at break of LDPE2040/TPS blends (27.5%glycerol in slurry);

FIG. 10 is the relative engineering maximum strength (smax/smax0) ofLDPE2040/TPS blends (27.5% glycerol in slurry).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing the present invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of construction and parts illustrated in the accompanyingdrawings and described herein. The invention is capable of otherembodiments and of being practiced in various ways. It is also to beunderstood that the phraseology or terminology used herein is for thepurpose of description and not limitation.

In general terms, the present invention provides novel compositions ofmatter including TPS/synthetic polymer blends having improved physicalproperties including tensile properties and increased TPS accessibilityover prior art achievements. The invention concurrently provides a novelprocess for achieving the new compositions.

As used herein, the expression starch refers to any starch of naturalorigin whether processed, chemically modified or treated, includingstarches such as for example: wheat starch, corn starch, potato starch,and rice starch. Starch can also be derived from plant sources such ascassava, tapica, and pea. It is a polysaccharide that consistsessentially of a blend of amylose and amylopectin.

Starch includes modified starches, such as chemically treated andcross-linked starches, and starches in which the hydroxyl groups havebeen substituted with organic acids, to provide esters or organicalcohols to provide ethers, with degrees of substitution in the range0-3.

Starch includes extended starches, such as those extended with proteins;for example with soya protein.

As used herein, the expression synthetic polymer refers to anysubstantially water-insoluble synthetic thermoplastic or thermosetmaterials. Examples of substantially water-insoluble thermoplastichomopolymer resins are polyolefins, such as polyethylene (PE),polypropylene (PP), polyisobutylene; vinyl polymers, such as poly (vinylchloride) (PVC), poly (vinyl acetate) (PVA), poly (vinyl carbazoles);polystyrenes; substantially water-insoluble polyacrylates orpolymethacrylates, such as poly (acrylic acid) esters, poly (methacrylicacid) esters; polyacetals (POM); polyamides, such as nylon6, nylon-6,6,aliphatic and aromatic polyamides; polyesters, such as poly(ethyleneterephthalate) (PET), poly(butylene terephthalate) (PBT);polyarylethers; polyurethanes, polycarbonates, polyimides, and highmolar mass, substantially water-insoluble or crystallizablepoly(alkylene oxides), such as poly(ethylene oxide), poly(propyleneoxide). As well as mixtures thereof.

Further included are polyesters and polylactides that are consideredbiodegradable in short time periods. Examples of those water insolublematerials are polylactones such as poly(epsilon-caprolactone) andcopolymers of epsilon-caprolactone with isocyanates; bacterialpoly(hydroxyalkanoates), such aspoly(hydroxybutyrate-3-hydroxyvalerate); and polylactides, such aspoly(lactic acid), poly(glycolic acid) and copolymers comprising therepetitive units of both; as well as mixtures thereof.

Further included are substantially water-insoluble thermoplasticalpha-olefin copolymers. Examples of such copolymers are alkylene/vinylester-copolymers as ethylene/vinyl acetate-copolymers (EVA),ethylene/vinyl alcohol-copolymers (EVAL); alkylene/acrylate ormethacrylate-copolymers preferably ethylene/acrylic acid-copolymers(EAA), ethylene/ethyl acrylate-copolymers (EEA), ethylene/methylacrylate-copolymers (EMA); alkylene/maleic anhydride-copolymerspreferably ethylene/maleic anhydride-copolymers; as well as mixturesthereof.

Further included are styrenic copolymers, which comprise random, block,graft or core-shell architectures. Examples of such styrenic copolymersare alpha-olefin/styrene-copolymers preferably hydrogenated andnon-hydrogenated styrene/ethylene-butylene/styrene copolymers (SEBS),styrene/ethylene-butadiene copolymers (SEB); styrene acrylonitrilecopolymers (SAN), acrylonitrile/butadiene/styrene copolymers (ABS); aswell as mixtures thereof.

Further included are other copolymers such as acrylic acidester/acrylonitrile copolymers, acrylamide/acrylonitrile copolymers,block copolymers of amide-esters, block copolymers of urethane-ethers,block copolymers of urethane-esters; as well as mixtures thereof.

Further included are thermoset resins such as epoxy, polyurethane,polyesters and their mixtures.

Further included are block or graft copolymers formed betweenhomopolymers and copolymers of hydroxyacids and one or more of thefollowing components:

-   -   (i) Cellulose or modified cellulose.    -   (ii) Amylose, amylopectin, or natural or modified starches.    -   (iii) Polymers resulting from the reaction, of a compound        selected from diols, prepolymers or polymers of polyesters        having terminal diol groups with monomers selected from the        group consisting of: bifunctional aromatic or aliphatic        isocyanates; bifunctional aromatic or aliphatic epoxides;        dicarboxylic aliphatic acids; dicarboxylic cycloaliphatic acids;        or aromatic acids or anhydrides.    -   (iv) Polyurethanes, polyamides-urethanes from diisocyanates and        amino-alcohols, polyamides, polyesters-amides from dicarboxylic        acids and amino-alcohols, and polyester-ureas from aminoacids        and diesters of glycols.    -   (v) Polyhydroxylate polymers;    -   (vi) Polyvinyl pyrrolidone, polyvinyl pyrrolidone-vinyl-acetate        copolymers and polyetheloxazoline. As well as mixtures thereof.

In the method and product of the present invention, the addition ofcompatibilizers or coupling agents can also be envisaged.Compatibilizers improve the adhesion at the interface and can beparticularly useful at further improving the properties at high loadingsof thermoplastic starch. The addition of an interfacial modifier stillallows for the obtention of highly continuous and co-continuous networkshowever the scale of said network becomes substantially finer.

Suitable compatibilizers or coupling agents for the TPS based blends canbe polymers or copolymers having functional groups that present specificinteractions with starch molecules and/or are capable of undergoingchemical reactions with starch functional groups to result in a chemicalbond. Those compatibilizers have preferably a low interfacial tensionwith the synthetic polymer, but more preferably a partial or fullmiscibility with the synthetic polymer. Examples of functional groupsthat present specific interactions and/or are capable to react withstarch are: Hydroxyl, carboxyl or carboxylate, tertiary amino and/orquaternary ammonium, sulfoxyl and/or sulfoxylate groups, and vinylpyrrolidone copolymers.

The compatibilizer having hydroxyl groups is preferably a polymercontaining vinyl alcohol units. More preferably it is a poly (vinylester) wherein the ester groups are partially hydrolyzed or a copolymercontaining vinyl alcohol units as well as other units such as areobtained by copolymerization of vinyl esters, preferably vinyl acetate,with monomers such as ethylene (EVOH), propylene, vinyl chloride, vinylethers, acrylonitrile, acrylamide, omega-octadecene, vinyl-butyl ether,vinyl-octadecyl ether, vinyl pyrrolidone and other known monomers, withsubsequent hydrolysis of at least some of the vinyl-ester groups.Preferred copolymers are e.g. poly (vinyl alcohol-co-vinyl-acetate);

ethylene/vinyl alcohol/vinyl acetate copolymers; ethylene/vinylchloride/vinyl alcohol/vinyl acetate graft copolymers; vinylalcohol/vinyl acetate/vinyl chloride copolymers; vinyl alcohol/vinylacetate/vinyl chloride/diacryl amide copolymers; vinyl alcohol/vinylbutyral copolymers; vinyl alcohol/vinyl acetate/vinyl pyrrolidonecopolymers; vinyl alcohol/styrene copolymers.

The compatibilizer containing carboxylic acid and/or carboxylate groupsis preferably a synthetic polymer, preferably a copolymer containingcarboxylate groups as well as other units such as are obtained bycopolymerization of acrylic acid, methacrylic acid, crotonic acid,maleic acid, itaconic acid, e.g. in their acid or carboxylate form, withmonomers such as ethylene, vinyl chloride, vinyl esters such as vinylacetate, vinyl ethers, acrylic acid esters, acrylonitrile, methacrylicacid esters, maleic acid esters, acrylamide, omega-octadecene,vinyl-butyl ether, vinyl pyrrolidone and other known monomers. If acarboxyl group-containing monomer is used for preparing the polymer,then at least a part of the carboxyl groups must be neutralized with acation. Preferred copolymers containing carboxylate groups are thosewhich can be described as being derived from acrylic acid, methacrylicacid, crotonic acid, maleic acid, itaconic acid, methylacrylate,methylmethacrylate, acrylamide, acrylonitrile and/or methylvinylether.More preferred polymers are those that can be described as being derivedfrom acrylic acid, methacrylic acid, maleic acid, methacrylate, ethylacrylate and/or methylvinylether. Such copolymers may be alsocopolymerized with ethylene, propylene, or styrene. Such copolymers are,e.g., poly (acrylic acid-co-vinyl acetate); ethylene/acrylic acid/vinylacetate copolymers; ethylene/vinyl chloride/acrylic acid/vinyl acetategraft copolymers; acrylic acid/vinyl acetate/vinyl chloride copolymers;acrylic acid/vinyl methylether copolymers; vinyl acetate/acrylicacid/acrylic acid methylester copolymer; vinyl acetate/crotonic acidcopolymers; vinyl acetate/maleic acid copolymers; methacrylic acid/vinylacetate/vinyl pyrrolidone copolymers; acrylic acid/acrylonitrilecopolymer; ethylene/propylene/acrylic acid copolymer; andstyrene/acrylic acid copolymer, wherein a part or all of the acid groupsare present in their carboxylate form. Copolymers that containcarboxylic groups are preferably copolymer of acids with ethylene, e.g.the ethylene-acrylic-acid copolymer in the form of its salt or anethylene-methacrylic acid copolymer in the form of its salt.

Compatibilizers which contain tertiary amino groups and/or salts thereofand/or quaternary ammonium groups are preferably a synthetic polymer, asobtained by the polymerization of monomers containing tertiary aminogroups and/or salts thereof and/or quaternary amino groups such as poly(2-vinyl pyridine); poly (4-vinyl pyridine); polyvinyl carbazole,I-vinyl imidazole and/or salts thereof and/or their quaternizedderivatives as well as with other polymers as are obtained bycopolymerization of such amines with other monomers such asacrylonitrile, butyl methacrylate, styrene and other known monomers. Theexpression amine salts includes the salts formed with an inorganic ororganic acid, e.g. salts with inorganic or organic acids such as HC1,H2SO4, and acetic acid. The expressions “quaternized derivative” and“quaternary ammonium groups” mean quaternized derivatives of tertiaryamines, e.g. quaternized with an alkyl halide such as methyl chloride.Preferred polymers are those derived from 2-vinyl-pyridine, 4-vinylpyridine and vinyl carbazole.

Compatibilizers having sulphonic acid and/or sulfonate functional groupsare preferably styrene sulphonic acid polymers, styrene sulfonic acidcopolymers, and salts thereof. More preferably they are block copolymersof sulfonated styrene with unsaturated monomers such as ethylene,propylene, butylene, isobutylene, butadiene, isoprene, and/or styrene.Preferred salts thereof, including the corresponding sulfonates aretheir salts with metal ions or the ammonium ion, preferably an alkalimetal ion, magnesium or zinc or NH₄ ⁺, preferably sodium, potassium orzinc, preferably the sodium salt.

Compatibilizers containing vinyl pyrrolidone are preferably copolymersof vinyl pyrrolidone with one or more monomers selected from the groupof vinyl esters, vinyl alcohol, allyl alcohol, ethylene, propylene,butylene, isoprene, butadiene, styrene, vinyl ethers, anddimethylaminoethyl methacrylate. Preferred are copolymers of vinylpyrrolidone with a monomer selected from the group consisting of vinylesters, vinyl alcohol, styrene and dimethylaminoethyl methacrylate.Preferred are further the poly (N-vinyl pyrrolidone-vinyl ester)copolymers and from these the poly (N-vinyl pyrrolidone-vinyl acetate)copolymers.

As used herein when referring to immiscible TPS/polymer compositions,the term “continuous” refers to either the TPS or the polymer phasebeing essentially constituted of a network of interconnected domains.The term “co-continuous” refers to a composition wherein both the TPSand the polymer phase are continuous. The expression “highly continuousTPS phase” refers to a composition where the TPS phase is dispersed inthe polymer phase and yet the TPS domains are nearly all interconnected.Highly continuous can be defined as the case where 50% or more of theTPS is extractable. The per-cent extractable TPS is based on the weightloss of TPS from a 1 mm length (machine-direction)×7.5 mm width(cross-direction) specimen subjected to hydrolytic degradation in asolution of HCl at 60 degrees Celsius for 96-150 hours. Extractedsamples were vigorously washed with distilled water and dried at 60degrees Celsius in a vacuum oven for 48 hours prior to weightmeasurement. The concept of continuity of the TPS phase is of particularimportance when measuring the biodegradability of a material. If the TPSphase is not continuous or highly continuous, the TPS domains will beencapsulated by a non-degradable polymer rendering the majority of theTPS phase substantially less accessible to biodegradation.

As used herein, the term “plasticizer” refers to any suitableplasticizer for achieving a TPS. Plasticizers include for example:adipic acid derivatives, such as tridecyl adipate; benzoic acidderivatives, such as isodecyl benzoate; citric acid derivatives, such astributyl citrate; glycerol itself and derivatives; phosphoric acidderivatives, such as tributyl phosphate; polyesters; sebacic acidderivatives, such as dimethyl sebacate; urea.

The plasticizer can be selected from the group consisting of glycerin,ethylene glycol, propylene glycol, ethylene diglycol, propylenediglycol, ethylene triglycol, propylene triglycol, polyethylene glycol,polypropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol,1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,5-hexanediol, 1,2,6-hexanetriol, 1,3,5-hexanetriol, neopentyl glycol,trimethylol propane, pentaerythritol, sorbitol, and the acetate,ethoxylate, and propoxylate derivatives thereof.

Moreover, the plasticizer can be selected from the group consisting ofsorbitol ethoxylate, glycerol ethoxylate, pentaerythritol ethoxylate,sorbitol acetate, and pentaerythritol acetate.

The plasticizer is present in a amount of from 1% to 50 wt %, butpreferably 5% to 50wt % and most preferably 100% to 35 wt % calculatedon the total weight of the final composition. When expressed in term ofwt % in the initial slurry, the plasticizer was present in a proportionof about 10 to about 32 wt %.

All the references to % glycerol in the reported data refer to theamount of glycerol in the initial slurry (TABLE 1). In the case wherewater is substantially eliminated from the TPS, the % glycerol (or anyother plasticizer) in the blend material after extrusion can becalculated from the slurry concentrations in TABLE 1 based on thefollowing calculation. For example, in a final extruded blend productcontaining 53% LDPE/47% TPS prepared from a 48.5% starch/27.5%glycerol/24% water suspension, the per-cent glycerol in the extrudate isabout 19%. If water is present at substantial levels in the final blendproduct, its weight will also need to be taken into account.

Description of the Method of Preparing the Novel Compositions

The method of the present invention uses a starch suspension as a firstfeed material and a synthetic polymer as a second feed material. Thesynthetic polymer is preferably ground into granules for ease of meltprocessing through a screw-type blender-extruder.

Referring now to FIGS. 1 and 2, there is shown a preferred embodiment ofthe extrusion apparatus used to carry-out the method of the invention.Referring to FIG. 1 a, an upper view of the extrusion system 10 shows atwin-screw extruder (TSE) 12 to which is attached a single-screwextruder (SSE) 14. In sharp contrast with the prior art, thethermoplastic starch (TPS) is prepared in the TSE 12 while the syntheticpolymer, in this case low density polyethylene (LDPE), is melted in SSE14. This method will be further described hereinbelow.

Preparation of the Starch Suspension

Wheat starch was mixed in different proportions with water and glycerol.During the starch extrusion, water is important to promote thegelatinization process. Once gelatinized, the glycerol plasticizesstarch. In addition to plasticizing starch; glycerol decreases theviscosity of TPS. In the suspension, the starch content varied from 48to 50% by weight. Water and glycerol were varied from 20% to 30% andfrom 32% to 19% by weight, respectively. The glycerol concentration wasvaried in order to achieve TPS of varying and controllable viscosities.The water content was modified to maintain a constant liquid/solid ratioof about 1:1 v/v. Three examples are reported in Table 1 below. Allcontents are expressed in terms of %/wt of suspension.

TABLE 1 Glycerol Example Starch content* content* Water content* 1 48 3220 2 48.5 27.5 24 3 50 20 30 *In the initial slurry

In a typical suspension, 640 g of glycerol was mixed with 400 g ofdistilled water and placed in a recipient. 960 g of starch powder waspoured in the recipient containing water and glycerol and stirred togive a homogeneous slurry. The slurry, once made, was used immediatelyin the preparation of LDPE/TPS blends. Starch suspensions aresusceptible to the problem of sedimentation. Furthermore, the viscosityof the starch suspension increases with time. This increase has beenattributed to the solvation of starch molecules and furtherre-arrangement into a gel-like structure. For these reasons the starchsuspension must be used as fresh as possible, especially if theviscosity affects the feeding rate.

Feeding the mixture to the extruder as a slurry is a novel approach topreparing these materials and ensures that the starch is fullydestructurized and that the glycerol is well dispersed throughout thestarch material. Both of those elements are necessary components toachieving blends with the high elongational properties achieved by thepresent invention.

One-Step Extrusion Process

a) Basic Setup

Blending was carried out in a one-step process. A single-screw extruder(SSE) 14 was connected to an intermediate zone of a co-rotatingtwin-screw extruder (TSE) 12 using a leak-proof adapter. The schematicrepresentation of the upper and side views of the extrusion system areshowed in FIGS. 1 and 2, respectively. This one-step approach allows forthe melt-melt mixing of the components which improves the morphologycontrol of the dispersed TPS phase. It also provides the possibility ofminimizing the contact time between the two polymers at high temperaturewhich is the principal parameter for controlling the thermal degradationof TPS. The single screw used was from C. W. Brabender Instruments(LID=26, length=495 mm, and compression ratio=2). The twin-screw was aLeistritz AG (LM 30.34), L/D=28, and length=960 mm. The above describedsetup allows for the separation of the different processes occurring inthis operation. Accordingly, the melting of LDPE takes place in SSE 14,while both the starch gelatinization and plasticization (SGP) and meltblending occur in TSE 12. The mixing of TPS and PE occurs in the latterhalf of TSE 12. For ease of description, TSE 12 is pictorially dividedinto zones 16 to 30 as the blending progresses.

b) TPS Preparation

An important feature of the present method is the preparation of the TPSwhich comprises the steps of starch gelatinization and plasticization(SGP). The screw configuration in TSE 12 was chosen to give a longenough residence time, which permits SGP in the first zones of TSE 12.SGP took place over three sub-sections of TSE 12: feeding section 16,SGP sections 18 and 20 and water extraction section 22. The starchsuspension was fed at a temperature lower than 25° C. in the firstsection of TSE 12. This zone was water-cooled in order to maintain a lowtemperature. SGP was carried out in the sections 18 and 20 of the TSE12. Due to the thermal instability of starch, SGP was carried out at 70°and 90° C. in the sections 18 and 20, respectively. Several kneadingsections were used to homogenize the resulting TPS. Back-flow kneadingelements were also adapted to increase the residence time and,consequently, ensure the complete destructuring and the homogeneity ofthe TPS. It also served to increase the extrusion pressure before theventing zone 22. Water extraction took place in section 22 of TSE 12.Low-pressure elements, a higher temperature (110° C.) and vacuum werefound to improve the water extraction. The venting zone 22 was connectedto a condensation system, which avoided the passage of volatiles throughthe vacuum line. Once the TPS is substantially water-free, it can beblended with the second polymer, in this case LDPE.

The flow rate of the extruded TPS had an influence on the pressureexerted by the starch and its final appearance. In order to study thisphenomenon, an TSE extruder configuration using just five zones wasused. This configuration was similar to the original eight zonesconfiguration, but zones 24, 26, 28 and 30 were taken out. Threecapillary dies were used to measure the viscosity of TPS. The flow rateof the starch suspension was compared to that of TPS at the exit of thecapillary die. Surprisingly, the difference between both flow rates wasalmost equal to the water content in the starch suspension. Likewise,TGA measurements indicated that the water content in TPS was around 1%.This approach is thus very effective in removing the water fromthermoplastic starch. This is a critical point since excess water givesrise to bubbles in the resulting starch/polymer blend. These bubbles notonly affect aesthetics but also diminish the mechanical properties ofthe blend. As such, TPS will be considered as a binary system composedof starch and glycerol.

In studying the effect of flow rate of the starch suspension on thequality of the extrudate, lower and upper limits of feeding were found.The lower limit was imposed by the increased residence time of the TPS.It is well known that the TSE works better under starve-fed conditions.In such a situation, the residence time is controlled by the screwconfiguration, the flow rate and the screw speed. The screw speed wasmaintained constant at 150 rpm in the whole series of melt mixing andviscosity measurement experiments. Evidence of degradation was found atflow rates of the extruded TPS lower than 20 g/min. The appearance ofTPS changed from a transparent and flexible material to a yellowish morerigid one. When the flow rate of TPS was lower than the mentioned limit,an unexpected increase in the pressure was also monitored. At higherflow rates, the pressure was proportional to the measured flow rate. Theupper limit for the flow rate of TPS was imposed by the water extractionin the venting zone 22. Problems of foaming were observed at flow ratesbetween 45-50 g/min of TPS. In contrast to the lower limit, the pressureexerted by the foamed TPS decreased as the flow rate increased. Bothphenomena were produced by the presence of water in the extrudate. Watervapor, at 150° C. was responsible for the foaming of TPS. Moreover,water excess reduced the viscosity of TPS in the extruder. This upperlimit can be overcome by the addition of another venting zone or themodification of the existing one with more efficient equipment. As ismentioned above, the flow rate, temperature, and screw design areimportant parameters to control.

c) Mixing

The blend mixing section can be divided into three sub-sections: LDPEaddition zone 24 mixing zone 26 and 28 and pumping zone 30. Thetemperature of the whole mixing section was maintained constant at 150°C. As observed in FIG. 1 a, the LDPE addition zone 24 has no heatingelement, however, the temperature was maintained around 150° C. by theconvection heating of the neighboring zones 14 and 26 and the moltenLDPE. The melt mixing of LDPE and TPS starts in zone 24. The melt mixingcontinued through the next two zones 26 and 28 aided by several kneadingand mixing elements. The pumping zone 30 is necessary to pressurize theextrudate through the die head.

The proportion of thermoplastic starch in terms of wt % of the resultingTPS/polymer blend was about 10 to 60 wt %, and preferably about 20 to 55wt %.

It is to be noted that by attaching the single-screw extruder 14progressively downstream (zones 26, 28 or 30) on the twin-screw 12 it ispossible to achieve the same level of morphology control as reportedhere at very low blend residence times. Thus, one of the advantages ofthe single step approach is that it can be used to minimize theresidence time of starch in contact with a high melting polymer.Therefore, TPS can be blended with high melting temperature polymerssuch as PP, PS, PET etc. while still minimizing thermal degradation ofthe starch.

The die head 32 and SSE 14 were operated at the same temperature as themixing section. The screw speed of SSE 14 was kept constant using anarbitrary measure of the motor speed (2.5) and the flow rate of LDPE wascontrolled with the aid of a pellet feeder. Maximum pumping of SSE 14under these conditions was 100 g/min.

d) Sheet Take-Up

LDPE/TPS blends were extruded through a rectangular die. Blends werequenched using calendar rolls. Calendar rolls were used because blendscould not be quenched in cold water due to the highly hydrophilic natureof TPS. The strain ratio, the ratio between the speed of extrudate andthe speed of the ribbon at the exit of the calendar, was around 2. Thatimposed a machine direction deformation on the ribbon. The morphology ofthose blends showed evidence of that deformation. The evolution of themorphology of LDPE/TPS blends are reported below.

Novel TPS/Polymer Compositions

The morphology of LDPE/TPS blends prepared in accordance with the methodof the present invention was studied using a scanning electronmicroscope (SEM). LDPE/TPS blend ribbons were cryogenically fractured toobtain surfaces both axial and transversal to the machine direction.Fractured samples were coated with gold palladium alloy and furtherobserved in a JSM-820 SEM.

a) Influence of the Glycerol Content

Micrographs taken in the axial direction (machine direction) of PEblended with ca. 30% of TPS and compounded with either 20% or 27.5% ofglycerol is shown in FIG. 2. The particle diameter of TPS domainscompounded with 20% glycerol was larger than that of JPS having higherglycerol content.

Furthermore, TPS (20% glycerol) domains demonstrated only a slightdeformation even though all blends were quenched at similar take-upspeeds. This could be the consequence of the higher viscosity of TPScompounded with only 20% glycerol. The viscosity of polymers compoundedwith low molecular weight plasticizer decreased as the plasticizercontent increased. Particles of TPS made with 20% glycerol wereelliptical with a minor axis diameter ranging between 10 um to >50 um.That means that those particles were larger than the original granularstarch. This is surprising considering that TPS has been completelygelatinized and plasticized. It seems that the viscosity of the two LDPEtypes tested was not high enough to disintegrate TPS particlescontaining 20% glycerol into a smaller dispersed phase. However, blendsprepared with LDPE2040, the PE having the lower melt flow index,demonstrates a finer particle size than that of LDPE2049. On the otherhand, the TPS compounded with the larger quantity of glycerol wasdeformed into fiber particles by both PIE matrix materials.

b) Influence of the LDPE/TPS Concentration Ratio

The evolution of the TPS domains as a function of composition inLDPE/TPS blends in the axial direction is shown in FIG. 3. It isimportant to note that the glycerol content (based on the slurry) inthis TPS was 27.5%. The fiber-like structures (found throughout thethickness of the sheet) are a result of the high concentration of TPSand are also due to deformation processes experienced in the die and asthe material exits the die. This structure is preserved by quenchingcalendar rolls. The fiber diameter increased from 2-4 um to >10 um asthe concentration of TPS increased from 29% to 53.5% TPS. TPSfiber-fiber coalescence is evident at TPS concentrations of 35.5% ormore. The morphology of LDPE/TPS blends fractured in the transversedirection revealed that TPS domains were more strongly deformed in themachine direction, see FIG. 4. As observed in the axial viewmicrographs, the fiber diameter increased as the TPS concentrationincreased. However, evidence of coalescence was observed even at thelowest TPS concentration (29.0%). Coalescence of the TPS domainsoccurred to a very high degree at 53.3% TPS.

It is possible, if desired, to form a thin layer of LDPE at the surfaceof the product.

Accessibility of Thermoplastic Starch

Starch-based materials require that two important, and closely related,aspects be controlled: water absorption and biodegradability. Waterpermits the microorganisms to move and also helps them to metabolizestarch. Nevertheless, water may also affect the dimensional stability ofstarch-based materials and their properties. The present inventiontackles this problem by controlling the morphology of these blends. Thecontinuous structure allows for the accessibility of starch domains. Theaccessibility of starch domains in LDPE2040/TPS blends was studied. Thepercent extractable TPS is based on the weight loss of TPS from a 1 mmlength (machine-direction)×7.5 mm width (cross-direction) specimensubjected to hydrolytic degradation in a solution of HCl at 60 degreesCelsius for 96-150 hours. Extracted samples were vigorously washed withdistilled water and dried at 60 degrees Celsius in a vacuum oven for 48hours prior to weight measurement.

Blends of LDPE/TPS having higher glycerol contents showed a fiber-likeand nearly continuous morphology in the machine direction. Consequently,a higher accessibility in the axial direction was expected. In order todetermine the influence of such connectivity on degradability, sampleswere exposed to hydrolytic extraction, see FIG. 5.

In both, LDPE2049/TPS (20% glycerol) and LDPE2040/TPS (27.5% glycerol),the accessibility of starch domains increases with TPS concentration andreaches a maximum at the phase inversion region. TPS containing a highglycerol content of about 27.5 wt % was more accessible for starchextraction. This was unexpectedly achieved because of the fiber-likemorphology observed in the SEM micrographs. In blends containing morethan 45% by weight TPS, the starch phase has been completely extracted,that was an indication of a co-continuous morphology. Co-continuity isvery desirable for a maximum accessibility of the biodegradable portionin synthetic polymer/starch blends.

Tensile Properties

a) Machine Direction

LDPE/TPS blends were tested according to the ASTM D-638 method. Tensilespecimens of type V were cut longitudinally from LDPE/TPS ribbons.Samples were strained at 10 mm/min on a M30K machine (JJ Instruments)equipped with a 5 kN cell and a data acquisition system. The averagevalues of the Young's modulus, maximum tensile strength and elongationat break were calculated from at least 12 measurements.

The relative elongation at break (ε/ε₀) of LDPE/TPS blends is presentedin TABLE 2 and FIG. 6. In TABLE 2, ε, and ε₀ are the elongation at breakof LDPE/TPS blends and pure LDPE, respectively. Blends containing highand intermediate glycerol contents maintain a high machine directionelongation at break, modulus and strength even at high loading. In fact,the elongation at break of those blends are virtually the same as thepure polyethylene. In ductile synthetic polymer blends the high loadingof an immiscible second phase results in highly fragile materials. Thisoccurs because elongation at break is a parameter which is highlysensitive to the state of the interface. Immiscible TPS/PE blendsdemonstrate high machine direction tensile properties even in theabsence of an interfacial modifier. Improvement in the elongation atbreak of these blends is an important feature compared to prior artblends. This is probably due to the highly continuous nature of thedispersed TPS phase as well as the improved removal of water duringprocessing. In the prior art methods, TPS was blended with LDPE and thenpassed through the venting section. Since the dispersed TPS isencapsulated in an LDPE matrix, this led to impeded water removal. Thepresence of water at the processing temperature can produce bubbles inthe extrudate weakening the final product. In the present invention,water is extracted from TPS before mixing with polyethylene and theproblem of residual water is circumvented.

Blends having the lowest glycerol content failed at lower elongation.This phenomenon was more marked in the case of blends prepared withLDPE2049. The drop in the elongation at break of blends prepared withTPS compounded with 20.0% glycerol was expected because of the largersize and poor dispersion of starch particles in the LDPE matrix.

The relative Young's modulus and maximum tensile strength of LDPE (Ε₀)and LDPE/TPS blends (Ε) are shown in TABLE 2 and FIG. 7. The modulus andmaximum strength of LDPE/TPS blends compounded with high glycerolcontent decreases somewhat with TPS content. It is worth noting that the2040 LDPE/TPS blends with 27.5% glycerol maintain almost the samemachine direction modulus and maximum strength of polyethylene up to 35%TPS loadings. In contrast, the addition of TPS compounded with 20.0%glycerol augmented the modulus of LDPE. That modulus was less than forLDPE/granular starch composites.

b) Cross Direction

Microtensile cross direction properties are shown in TABLE 3 for samplesconditioned for 48 hrs at 0% and 50% humidity. At 29% TPS, the modulusand maximum tensile strength are maintained at 80 and 83% the level ofpolyethylene at 0% humidity. At 50% humidity the modulus and maximumtensile strength property retention is at 71 and 76% respectively. Theelongation at break is diminished more significantly, but under allconcentration conditions studied, the material remains highly ductile.Thus, even the cross properties in this material perform much betterthan that observed in typical immiscible synthetic polymer blends. Itmust be underlined that this blend material was prepared with a machinedirection melt draw ratio of about 2:1. This results in preferentialdispersed phase orientation in the machine direction. Thus it is normalthat the cross direction properties should be weaker. It is possible tosubstantially improve the cross direction properties by minimizing themelt-draw ratio. Machine direction orientation can also be reduced byreducing the glycerol content. FIG. 2 showed less elongation of the TPSphase in the md when 20% glycerol was used as compared to the 27.5%case. For the case of 20% glycerol where significantly less machinedirection elongation was obtained, the cross elongation at breakproperties improve substantially as shown in TABLE 3.

Table 3 clearly indicates that the cross direction modulus issubstantially increased at lower glycerol contents.

A number of parameters can be brought to bear in order to control theproperties of the blend system. Applying an axial draw ratio can be usedto modify the properties in the machine direction. Minimizing the axialdraw results in improved cross direction properties (particularly crossdirection elongation at break). The system still maintains highcontinuity even under those latter conditions. Reducing the per-centglycerol results in an increase in the modulus of the blends. It ispossible using the above parameters to tailor the material to a givenapplication.

c) Effect of Aging

FIGS. 8-10 demonstrate the properties in the machine direction for the27.5% glycerol preparation. Two cases are shown: the properties soonafter preparation (tested at about 50% humidity) and the propertiesafter one year (conditioned at 0% and 50% humidity for 48 hrs prior totesting). There is little effect of aging on the modulus, maximumtensile strength and elongation at break.

d) Effect of Humidity

In order to evaluate the effect of short term exposure to humidity onmechanical properties, the samples were conditioned for 48 hrs in 0% and50% humidity environments as already mentioned above. The results forthe machine direction properties are shown in FIGS. 8-10 for the 27.5glycerol preparations. For that glycerol preparation there is verylittle effect of humidity on the machine direction elongation at breakand maximum strength. A small effect of humidity is observed on themodulus.

The cross direction properties shown in TABLE 3 demonstrate similartendencies as above for the 27.5% glycerol study. Very little effect ofhumidity is observed on the maximum strength and elongation at break.Some effect on the modulus is observed. TABLE 3 indicates that for the20% glycerol preparation, humidity also results in a substantialincrease in the elongation at break. At 20% glycerol no effect ofhumidity is observed on the maximum strength. An effect on modulus isalso observed at the 20% glycerol concentration.

Transparency

One of the very particular features of the novel compositions of thepresent invention is that 1 mm thick ribbons of this blend with as muchas 53% thermoplastic starch demonstrate a substantial level oftransparency.

Consequently, the results reported herein reveal LDPE/TPS blends, insheet form, with high loadings of TPS that maintain essentially the sameelongation at break in the machine direction as pure PE even in theabsence of interfacial modifier(s). The modulus and maximum tensilestrength are also maintained at high levels. Good cross-properties arealso obtained. These blends are prepared in a combinedsingle-screw/twin-screw one-step process under carefully controlledprocessing conditions (flow rate, temperature, screw design,devolatization) and glycerol content. The morphology can be controlledby the composition and processing conditions to yield a highlycontinuous or co-continuous structure. In such a case, nearly all of theTPS becomes accessible for biodegradation.

In addition, the method of the present invention was also tested onblown film production. These experiments provided a film material whichexhibited a high level of transparency even at high loadings of TPS.

Although the invention has been described above with respect to specificembodiments, it will be evident to a person skilled in the art that itmay be modified and refined in various ways. It is therefore wished tohave it understood that the present invention should not be limited inscope, except by the terms of the following claims.

TABLE 2 Mechanical properties of LDPE/TPS blends (ribbons) in themachine direction. Glycerol TPS in slurry σ_(max) σ_(max)/ ε_(b) EMaterial (%) (%) (GPa) σ_(max0) (%) ε_(b)/ε_(b0) (GPa) E/E₀ a). LDPE2040MFI = 20.0 g/10 min. PE2040 0.0 0.0 11.8 1.00 482 1.00 55.9 1.00 P0A331.4 32.0 9.7 0.83 465 0.97 44.0 0.79 P0A4 46.2 32.0 8.7 0.74 449 0.9344.4 0.79 P0A5 49.5 32.0 8.7 0.74 415 0.86 41.2 0.74 P0A13 29.0 27.510.5 0.89 464 0.96 60.0 1.08 P0A14 35.5 27.5 9.8 0.83 427 0.89 50.1 0.90P0A15 44.7 27.5 9.0 0.76 453 0.94 42.4 0.76 P0A16 53.3 27.5 8.2 0.69 3880.80 34.2 0.61 P0A23 29.8 20.0 9.8 0.83 400 0.83 60.2 1.08 P0A24 41.020.0 9.6 0.82 345 0.72 64.4 1.15 P0A25 48.9 20.0 8.2 0.69 230 0.48 66.21.19 b). LDPE2049 MFI = 12.0 g/10 min. PE2049 0.0 0.0 10.5 1.00 493 1.0058.6 1.00 P9A3 32.1 32.0 8.9 0.85 492 1.00 41.5 0.71 P9A4 34.9 32.0 9.00.86 403 0.82 41.6 0.71 P9A5 40.8 32.0 9.4 0.89 407 0.82 36.2 0.62 P9A1333.7 27.5 9.0 0.86 480 0.97 40.5 0.69 P9A14 36.9 27.5 8.1 0.76 429 0.8743.4 0.74 P9A15 38.2 27.5 9.2 0.88 430 0.87 41.9 0.71 P9A23 28.6 20.06.3 0.60 89 0.18 68.7 1.17 P9A24 33.0 20.0 5.9 0.56 34 0.07 75.4 1.29P9A25 46.3 20.0 6.1 0.58 65 0.13 71.8 1.23

TABLE 3 Mechanical properties of LDPE/TPS blends (micro-tensilespecimens) in the cross direction Glycerol TPS in slurry F_(max)F_(max)/ ε_(b) E Material (%) (%) (N) F_(max0) (%) ε_(b)/ε_(b0) (GPa)E/E₀ a). Conditioned at 0% relative humidity and room temperature.PE2040 0.0 0.0 26.5 1.00 220 1.00 43.3 1.00 P0A13 29.0 27.5 21.1 0.80 850.38 36.0 0.83 P0A14 35.5 27.5 19.1 0.72 62 0.28 35.0 0.81 P0A15 44.727.5 15.7 0.59 43 0.19 25.3 0.58 P0A16 53.3 27.5 13.8 0.52 33 0.15 24.50.57 P0A23 29.8 20.0 23.5 0.89 163 0.73 42.7 0.99 P0A24 41.0 20.0 22.60.85 84 0.38 40.7 0.94 P0A25 48.9 20.0 20.3 0.77 41 0.19 43.8 1.01 b).Conditioned at 50% relative humidity and room temperature. PE2040 0.00.0 25.9 1.00 328 1.00 44.1 1.00 P0A13 29.0 27.5 19.6 0.76 90 0.27 31.30.71 P0A14 35.5 27.5 18.1 0.70 71 0.22 27.0 0.61 P0A15 44.7 27.5 12.80.49 49 0.15 18.1 0.41 P0A16 53.3 27.5 9.3 0.36 39 0.12 14.0 0.32 P0A2329.8 20.0 24.0 0.93 454 1.38 33.8 0.77 P0A24 41.0 20.0 20.7 0.80 2560.78 30.8 0.70 P0A25 48.9 20.0 18.2 0.70 205 0.62 24.5 0.55

1. A method of preparing a thermoplastic starch and synthetic polymerblend, said method comprising the steps of: (a) providing a starchsuspension comprising starch, water and a plasticizer; (b) obtaining athermoplastic starch from said starch suspension by causinggelatinization and plasticization of said starch suspension by exertingheat and pressure on said starch suspension in a first extrusion unit;(c) venting off residual water from said thermoplastic starch to obtaina substantially moisture-free thermoplastic starch; (d) obtaining a meltof a synthetic polymer or polymer blend in a second extrusion unit; (e)combining said melt obtained from step (d) with said substantiallymoisture-free thermoplastic starch obtained from step (c) to obtain athermoplastic starch and synthetic polymer blend.
 2. A method inaccordance with claim 1 wherein said plasticizer is glycerol.
 3. Amethod in accordance with claim 2 wherein said glycerol is present in aproportion of about 10 to about 32 wt % based on the total weight of theslurry suspension.
 4. A method in accordance with claim 1 wherein saidsynthetic polymer is polyethylene.
 5. A method in accordance with claim2 wherein said synthetic polymer is polyethylene.
 6. A method inaccordance with claim 3 wherein said synthetic polymer is polyethylene.7. A method in accordance with claim 1 wherein said first extrusion unitis a twin-screw extruder.
 8. A method in accordance with claim 7 whereinsaid second extrusion unit is a single-screw extruder.
 9. A method inaccordance with claim 1 wherein step (b) is conducted at a temperatureof about 50 to about 100° C.
 10. A method in accordance with claim 1wherein step (d) is conducted at a temperature of about 70 to about 200°C.
 11. A method in accordance with claim 1 wherein said starch is wheatstarch.
 12. A method in accordance with claim 8 wherein said single-screw extruder is connected to said twin-screw extruder at essentiallya right angle relative thereto.